Here is an example from the logfile showing the series of events from when the USB flash drive is inserted, to when the system fails during a file copy (it can happen with a single 58MB file or with 2.1MB files whether they are dragged as files or in a folder)...
8/11/12 11:27:35.382 PM fseventsd[34]: log dir: /Volumes/C_USB_2GB/.fseventsd getting new uuid: 06CDAC3D-1BEF-4510-ADA7-DC978DDE13A5
8/11/12 11:28:20.000 PM kernel[0]: USBF: 91820. 35 AppleUSBEHCI[0xffffff80198d3000]::Found a transaction which hasn't moved in 5 seconds on bus 0xfa, timing out! (Addr: 0, EP: 0)
8/11/12 11:28:26.000 PM kernel[0]: USBF: 91826. 39 AppleUSBEHCI[0xffffff80198d3000]::Found a transaction which hasn't moved in 5 seconds on bus 0xfa, timing out! (Addr: 0, EP: 0)
8/11/12 11:28:26.000 PM kernel[0]: USBF: 91826. 71 [0xffffff80199cdc00] The IOUSBFamily is having trouble enumerating a USB device that has been plugged in. It will keep retrying. (Port 4 of Hub at 0xfa100000)
8/11/12 11:28:32.000 PM kernel[0]: USBF: 91832. 44 AppleUSBEHCI[0xffffff80198d3000]::Found a transaction which hasn't moved in 5 seconds on bus 0xfa, timing out! (Addr: 0, EP: 0)
This was a drive formatted on the same MacBook Pro Mountain Lion a few minutes prior to this, so the problem is not in how this drive was formatted. It happens with other flash drives also.
A separate problem is unrelated to sleep.. Unexpected ejects during file copies without sleep involved.
Count me on these increasingly dangerous mid-data-transfer ejects, even of things that do not have fancy drivers (e.g. camera memory cards) since Mountain Lion. The problems are NOT limited to USB. They happen on FireWire 800 on my MacBook Pro as well as on USB with or without a hub.
Imran
Imran